Compressed air supply assemblies are used in vehicles of all types to supply vehicle air spring assemblies with compressed air. Air spring assemblies can comprise level control devices that can be used to adjust the distance between the vehicle axle and vehicle superstructure. An air spring assembly of such a pneumatic system comprises a number of air bellows pneumatically connected to a common line (gallery) and can raise the vehicle superstructure as fill levels increase and lower it as fill levels decrease. As the distance between the vehicle axle and the vehicle superstructure, or the ground clearance, increases, the spring travel becomes longer and greater ground unevenness can be overcome without contact with the vehicle superstructure. Such systems are used in off-road vehicles and sport utility vehicles (SUVs). In SUVs with very powerful engines, it is desirable to provide the vehicle with comparatively low ground clearance for high speed road travel, and with comparatively high ground clearance for off-road use. It is furthermore desirable to implement the change in ground clearance as quickly as possible, which increases the requirements with regard to speed, flexibility and reliability of a compressed air assembly.
A compressed air supply assembly for use in a pneumatic system with a pneumatic assembly, for example an air spring assembly of the general type discussed above, is operated with compressed air from a compressed air supply, for example within a pressure range from 5 to 20 bar. The compressed air is provided to the compressed air supply by means of an air compressor, for example a compressor or where applicable a double compressor. To supply the pneumatic assembly, the compressed air supply is pneumatically connected with a compressed air connection and on the other side is pneumatically connected with a purge connection. Via a solenoid valve arrangement forming a purge valve arrangement, by allowing the air to escape into one or more purge connections, the compressed air supply assembly can be purged to the environment.
Providing an air dryer to dry the compressed air supplied to the system ensures long-term operation of the compressed air supply assembly. That is, the air dryer prevents the accumulation of moisture in the pneumatic system, which otherwise could lead to valve-damaging crystal formation at comparatively low temperatures and also to undesirable defects in the compressed air supply assembly and in the pneumatic assembly. An air dryer has a drying medium, normally a granulate bulk product through which the compressed air can flow so that, at comparatively high pressure, the granulate can absorb the moisture contained in the compressed air. An air dryer can, where applicable, also be designed as a regenerative air dryer, whereby, in each purge cycle, at comparatively low pressure, the dried compressed air from the air suspension system can flow through the granulate in the reverse direction or in the same direction relative to the filling direction. For this, the purge valve arrangement can be opened. For such an application, also known as pressure change adsorption, it is desirable to design the compressed air supply assembly to allow a comparatively rapid purging but nonetheless with a pressure change sufficient for regeneration of the air dryer. It is also desirable to carry out the purging process for a compressed air supply assembly with as high a purge pressure amplitude as possible, i.e., from an operating pressure to a purge pressure. However, the components of the solenoid valve arrangement should not be over-dimensioned or made too complex in construction.
So-called direct purge circuits—without control valve—can render superfluous the functionality of a solenoid valve arrangement of the type discussed above with control and purge valve. However, in such direct purge circuits, the nominal width of the purge valve must be designed to be comparatively large to allow rapid purging, but, at the same time, it is limited by the current absorption of a magnetic coil for the purge valve, so that, in the end, only limited switching pressure differences can be achieved.
EP 1 165 333 B2 describes a compressed air supply assembly of the general type discussed above, which comprises a solenoid valve arrangement with a control valve to control a purge valve. The control valve is connected with a valve connection in a control line that is connected to a pressure control connection of the purge valve. The purge valve is connected with a valve connection in the purge line. These and similar compressed air supply assemblies in principle allow rapid purge switching. The control valve is typically a 3/2-way valve and is switched by pressure provided by an air bellows of the pneumatic main line or by a control pressure taken from this. The control pressure, which can thus be delivered to the pressure control connection of the purge valve, actuates the opening of the valve connection of the purge valve in the purge line and thus initiates the purging of the compressed air supply assembly.
However, a disadvantage of compressed air supply assemblies that follow such a basic concept—such as for example the assemblies described in DE 102 23 405 B4 or EP 0 978 397 B1 or DE 10 2009 029 898 A1—is that a relay piston of the purge valve normally requires a closure-maintaining force provided by a valve spring, so that a residual pressure—and corresponding pre-control pressure—must be reserved in the system for the relay piston of the purge valve. The closure-maintaining force of the valve spring is required, for example, to enable the air compressor to deliver against comparatively high pressures. Also, the opening behavior of the purge valve must be taken into account for a residual pressure. These circumstances regularly lead to the reservation of a minimum bellows pressure required to constitute a pre-control pressure, for example in the range of one or a few bar, in order to be able to provide a fundamental purge functionality of the solenoid valve arrangement. Residual pressures can lie in the range from 1 or 2 to 3 bar, so that in a design of the compressed air supply assembly, a lower pressure limit for a bellows pressure of 3 bar or more must be defined. Despite such a design, depending on operating circumstances, partly closed valve cross sections can occur; these in turn can lead to a significantly reduced regeneration behavior in an air dryer and hence to unstable system operating states.